Existential crisis

[avram]

Horrifying news: the kilogram is evaporating! That article explains how the kilogram is defined as the weight of a particular cylinder of platinum and iridium, kept in a chateau near paris, and how it’s apparently lost 50 or so micrograms since it was made in 1889, and this could eventually lead to the destruction of the universe. Very amusing, though the author confuses weight with mass.

I figured he was wrong. I mean, we currently define weights and measures by reference to things much more durable than physical object, right? Well, in some cases, yes. Here’re the definitions of meter and second, from the International Bureau of Weights and Measures:

meter

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.

second

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

There’s some defining for ya! Now look at the base unit of mass:

kilogram

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

What’s the deal with that?

Comments

They're working on it

[redbird]

The deal with that is that, when the metric system was first instituted, actual physical standards were used as much as possible. There is a no-longer-canonical piece of metal with two marks, one meter across, which used to be the official definition of the meter. (Just as well: the meter was intended as one ten-millionth of the distance from the North Pole to the equator, but the measurements were off: if the meter hadn't been defined by that object in a vault, a lot of things would have had to be recalibrated.)

It's turning out that defining mass in unambiguous terms without referring to a single physical standard is harder than it seems; at least two teams are working on doing so.